In Tokkai Sho 53-19729 published by the Japanese Patent Office in 1978, there is disclosed a purge mechanism in which fuel evaporated inside a fuel tank of an automobile engine is sucked into a canister, and then supplied from the canister into an intake passage of the engine.
When the pressure in the fuel tank has risen above a predetermined value, this purge mechanism feeds the evaporated fuel in the fuel tank to the canister, in which it is adsorbed by an adsorbent material like activated charcoal. Provided that specified vehicle running conditions are satisfied, the adsorbed fuel is supplied via a purge conduit to the engine intake passage as a gas mixed with air. The flow rate of this purge gas is proportioned to the flow rate of the air in the engine intake passage.
The air-fuel ratio of the fuel mixture supplied to the engine is made richer by this purging, but this can be compensated for by detecting the air-fuel ratio with a sensor and by controlling the amount of fuel injected to the engine based thereon, i.e. by so called feedback control of air-fuel ratio. However, this type of feedback control device generally is arranged to control the air-fuel ratio so as to keep t in the vicinity of the theoretical or stoichiometric value of 14.6, and when a vehicle incorporating a lean burn engine which is operated at a lean air-fuel ratio for example about 22, the air-fuel ratio is generally controlled by an open loop control method, without any feedback control. This is because oxygen sensors, which are widely used for detection of the air-fuel ratio from the oxygen concentration in the engine exhaust gas, can only detect changes of the air-fuel ratio in the vicinity of the stoichiometric value. If a sensor is used which can detect the air-fuel ratio over a relatively wide region of values thereof, it is possible to perform feedback control in the lean air-fuel ratio region as well; but the cost of this type of sensor is quite high.
In this connection, Tokkai Sho 61-87935 published by the Japanese Patent Office in 1986 discloses an air-fuel ratio control device which, during engine operation near the stoichiometric air-fuel ratio, performs feedback control for the amount of fuel injection, and which, during engine operation at a lean air-fuel ratio, performs open loop control for the amount of fuel injection by using a correction coefficient which is learned during the above described feedback control.
This device is able to control the air-fuel ratio in the lean operational region comparatively accurately without using any expensive wide range air-fuel ratio sensor.
However, even with this device, the problem remains that the air-fuel ratio enriches when engine operation at a lean air-fuel ratio and purging of evaporated fuel are performed in parallel.
As compared with operation near the stoichiometric air-fuel ratio, during operation at a lean air-fuel ratio the amount of fuel injected for the same amount of intake air is less. On the other hand, since the flow rate of the purge gas is controlled in proportion to the intake air flow rate as described above, the flow rate of the purge gas does not change if the flow rate of intake air does not change, even if the engine running conditions have changed from operation near the stoichiometric air-fuel ratio to operation at a lean air-fuel ratio. As a result, during lean engine operation, the amount of fuel supplied to the engine in the form of the purge gas is relatively increased in relation to the fuel amount of the injection. Accordingly, when the correction coefficient which was learned during engine operation near the stoichiometric air-fuel ratio is utilized for control during operation at a lean air-fuel ratio, the actual air-fuel ratio undesirably becomes richer than the target air-fuel ratio. Such enrichment of the air-fuel ratio during lean engine operation exerts an undesirable influence upon the drivability of the vehicle and upon the composition of the exhaust gases thereof.
It is possible to compensate for this tendency to enrichment by performing feedback control of lean air-fuel ratio using a wide range air-fuel ratio sensor. However, even in this case, since the amount of correction provided by such feedback control can become quite large, a certain time period is required before the air-fuel ratio reaches its target value, when switching over from operation at the stoichiometric air-fuel ratio to operation at lean air-fuel ratio.